1. Field of the Invention
This invention relates to a high quality synthetic diamond single crystal substantially free from crystal defects, strains, etc., which can be applied to uses such as optical parts, spectroscopic crystals, monochromators, laser windows, anvils, semiconductor substrates, heat resistance semiconductors, device substrates for large power, and to a process for the production of the same.
2. Description of the Related Art
Diamond crystals can be applied to various uses such as heat sinks, dies for wire drawing, cutting tools for precision working, optical parts, laser windows and anvils for producing ultra-high pressures, because they have high hardness, high strength, excellent thermal conductivity, excellent corrosion resistance and good transmittance of lights.
Naturally occurring diamonds, most of which are called type Ia, contain about 1000 ppm of nitrogen. The nitrogen in this natural diamond is distributed in the crystal in an aggregated form, so that crystal defects and internal strains are large which results in the absorption of light due to nitrogen in the infrared range. Depending upon the variety of a rough stone, there is a large dispersion. Thus, the applied use has been limited to heat sinks or tools.
High purity natural diamond containing nitrogen impurity in an amount of at most several ppm is called type IIa and such diamond is very rare as represented by, for example, an output of about 2% based on the whole rough stones. Since the natural type IIa diamond contains very small amount of impurities, is colorless and transparent and has superior transmittance property, it has widely been applied to jewels, optical parts and laser window materials.
However, there remain defects or strains to a great extent in the interior part of the natural diamond, because of complicated growth progresses in the interior part of the earth. Accordingly, the natural diamond cannot be applied to uses requiring high crystallinity, for example, monochromators or semiconductor substrates. Furthermore, the natural diamond of type IIa has a problem that the output quantity is small resulting in higher production costs and difficulty in obtaining a supply of such diamonds.
An ordinary diamond artificially synthesized under an ultra-high pressure and high temperature is called type Ib and contains several hundreds ppm of nitrogen. Since this nitrogen is contained in the diamond crystal as an isolated substitutional impurity, absorption of light due to the nitrogen impurity occurs in the infrared range and ultraviolet range and the diamond crystal cannot be applied to optical parts or window materials. In addition, the distribution of nitrogen is largely uneven in the interior part of the crystal, thus resulting in more defects or strains in the crystal. The number of needle-shaped defects in the diamond single crystal is at least 10.sup.6 pieces/cm.sup.2.
On the other hand, it is known that when a nitrogen getter such as Al or Ti is added to a solvent metal during synthesis of the diamond, the nitrogen in the diamond can be lowered to about several ppm. When the nitrogen getter is added to the solvent metal, however, inclusions in large amounts ordinarily tend to be taken in the crystal which largely decrease the production yield of a good quality crystal. Accordingly, the production cost of the synthetic type IIa diamond is higher than that of the natural type IIa diamond and as a result the production thereof has not been carried out on a commercial scale.
On the contrary, the inventors have found that an inclusion-free synthetic type IIa diamond having a nitrogen content of at most 0.1 ppm can stably be synthesized by using at least one element selected from Group IVa and Va elements having a high nitrogen removal efficiency as a nitrogen getter, which limits inclusions from being taken in the crystal.
Up to the present time, diamond has widely been used as jewels, grinding abrasives, heat sinks, sound vibration plates, etc. In addition thereto, of late, crystals for spectroscopy in the X-ray range or semiconductor diamond substrates have been highlighted as new uses. In these uses, diamond is used under such a state that the excellent properties of diamonds are raised to the utmost limits and to this end, a low defect density diamond crystal is required. The low defect density diamond is composed less in natural crystals. Since it has been reported that diamonds artificially synthesized at a high pressure, in particular, diamond called type IIa, containing a very small amount of impurity, nitrogen, has less defects, the above described artificial diamonds have often been used in such fields needing the low defect diamond. However, at the present time when such low defect density diamond is being required, a supply of these artificial diamonds with relatively low defects and having such crystallinity is very rare and thus expensive.
On the other hand, it is known that diamond is after-treated under various states so as to improve the quality of the diamond. For example, as an after-treatment of diamond crystal grains used as abrasives for grinding diamond wheels, it has been proposed to improve the surface of raw material abrasives by a hydrogen plasma treatment as disclosed in Japanese Patent Laid-Open Publication No. 107088/1987 or to improve the toughness by separating and removing inclusions and then heating at a reducing atmosphere as disclosed in Japanese Patent Laid-Open Publication No. 165494/1995.
For use of jewels, it is known that defects are incorporated in a crystal by irradiation of rays, neutron, electron, etc. and then subjected to a heat treatment at a temperature of 300 to 800.degree. C. to impart various colors thereto "Genshi-Ryoku Kogyo (Atomic Energy Industry)", Vol. 40 (1994), No. 7, page 71!. It is assumed that these treatments serve to restore a slight shift of carbon atom from the definite position by irradiation of electron ray. In the case of a large shift of carbon atom from the definite position, exemplified by strain, however, there is no example of the restoration.
On the other hand, there are some reports for the purpose of changing crystallinity for uses of electronic materials. For example, Japanese Patent Laid-Open Publication No. 277176/1990 describes a method comprising forming a ring crack on the surface of a diamond crystal by an indentation member of a breakage tester, then subjecting it to a hydrogen plasma treatment at 600.degree. C. and thus providing a number of crystal defects on the diamond surface to increase the emission intensity; Japanese Patent Laid-Open Publication No. 24990/1993 describes a method comprising subjecting a diamond crystal to a hydrogen plasma treatment after an oxygen treatment and thus improving the interface with a metal formed thereon to form a good Schottky contact; Japanese Patent Laid-Open Publication No. 144995/1994 describes a method comprising heating a diamond crystal in an oxygen atmosphere and thus removing a chief cause of hindering the insulation without damaging the diamond; and Y. Mori: "Jpn J. Appl. Phys.", Vol. 31 (1992), L 1191 describes a method comprising implanting ion in the surface of a diamond crystal and then subjecting to a hydrogen plasma treatment to restore the breakage of the crystal when implanting ion. These examples aim at modifying only the surfaces of crystals and there is no example concerning improvement of crystallinity in the whole of a crystal.
Furthermore, it has been reported that diamond is deformed by heat-treating the diamond at a high temperature and high pressure, e.g. at least 7 GPa and at least 1000.degree. C. It is known that carbon atoms making up diamond can sufficiently be removed according to this method.
In the treatment at such a high pressure, however, there arises a problem that not only an expensive high pressure apparatus is required, but also defects are again incorporated into a diamond crystal during the course of lowering the pressure and temperature to normal pressure and temperature.
Natural diamond has a number of defects or large strains in the interior part of the crystal. Natural type IIa diamond contains less impurities, but is not good as to the crystallinity such as defects, strains, etc. Thus, the natural type IIa diamond has a problem that it tends to be cracked during use and when applying to technical fields requiring the strength of such diamond, for example, an anvil for producing an ultra-high pressure, compression cell for FT-IR, laser window material, etc., it is readily broken in some cases. Further, it cannot be applied to a field reauiring high crystallinity, for example, monochomaters, semiconductor substrates, etc.
On the other hand, a synthetic diamond, in particular, single crystal synthesized by the so-called temperature gradient method is far superior in crystallinity than natural diamond. The crystallinities of various diamond were estimated by the FWHM (full width at hall maximum) of an X-ray diffraction rocking curve using Cu--K.alpha. as a source to obtain results as shown in the following Table 1.
TABLE 1 ______________________________________ FWHM of Rocking Curve of Various Diamonds FWHM Quantity of Nitrogen of Rocking Curve Diamond (ppm) (arcsec) ______________________________________ Natural type Ia Diamond .about.1000 7-60 Natural type IIa Diamond &lt;1 200-2500 Synthetic type Ib Diamond 10-120 6-20 Synthetic type IIa Diamond &lt;0.1 4-6 ______________________________________
Herein, FWHM of a rocking curve is obtained by measuring an X-ray diffraction intensity curve (rocking curve) by the double crystal method using a synthetic diamond crystal (004) as a first crystal and searching for FWHM of this curve. If there are a number of defects or strains in a crystal, the rocking curve is broadened and thus a small FWHM shows excellent crystallinity. It is apparent from Table 1 that both natural type Ia and type IIa diamonds have considerably more defects or strains and synthetic diamonds (synthesized by temperature gradient method) exhibit much higher crystallinity. Of synthetic diamonds, the synthetic type IIa can stably be obtained with high crystallinity because of being free from defects or strains due to nitrogen impurity.
Even in the case of the type IIa diamond synthesized by the temperature gradient method, however, the crystallinity is not complete and observing defects in the crystal by X-ray topography, a number of needle-shaped dislocation defects are found. The defect is one characteristic of the synthetic diamond, which also appears often in the synthetic type Ib diamond by the ordinary temperature gradient method. Because of the crystal defect, even the synthetic type IIa diamond cannot be applied to uses needing high crystallinity such as monochromators, semiconductor substrates, etc.